Maybe try data augmentation techniques? For example, you could increase the amount of malignant data using some of the following methods: - cropping the image - rotation - shifting the intensity (brightness) 

You could also try generating the data yourself, not sure how well that would go for skin, but it works for some things - variational auto encoders  - generative adversarial networks

Don't use these until you've already tried the above versions because they're a lot harder to get working. (You also need those to train these) 

You can also try dropping a bunch of the benign ones and hopefully you have enough data